^Because Marlena thinks it's what is best. That and she committed to this plan over a month ago before John started having issues. She knows this John. He may get better and may try to manipulate the situation to his advantage. They've had Marlena explain numerous times her position. She feels like it's self-destructive for both of them to remain together. She's given John plenty of chances and while she doesn't want to divorce him, she feels it's what is best for both of them. She needs to take control of her life and she thinks it's best for him to be away from her. For months, she kept pushing him and, as a result, he did horrible things to form his own identity...to show he wasn't her John. She knows that and that is why she wants out. This way she won't be constantly reminding him of her John and he can live his own life without worrying about the ghost of old John.

She still loves him and that is why she's committed to helping him but she doesn't want to allow herself to give into him again so she's maintaining her divorce position. She doesn't want to get hurt again and she doesn't want to hurt him.

I already watched Monday too but I will wait to comment until tomorrow. My posts end to spoil the episode LOL.